Ethanol production through anaerobic fermentation of a carbon source by the yeast Saccharomyces cerevisiae is one of the best known biotechnological processes and accounts for a world production of more than 35 billion liters per year. Two-thirds of the production is located in Brazil and in the United States with the primary objective of using ethanol as a renewable source of fuel. Hence, there are strong economic incentives to further improve the ethanol production process. The price of the sugar source or carbohydrate source is a very important process parameter in determining the overall economy of ethanol production. Using unaltered yeasts, the greatest yield obtainable is only about 51.1%, with the remainder being lost to yeast maintenance and growth, glycerol production, and other end products. The typical ethanol yield is lower than the above-described maximum theoretical yield in large part due to competing microorganisms.
A typical ethanol production plant comprises a premixing vessel where water and the carbohydrate fuel source (hereafter referred to as mash) are held at 40° C. to 60° C. and where (if corn is the source of carbohydrate) a small amount of enzyme such as a-amylase is added. The mash is then heated to between 90° C. to 150° C. for a period of time, and then cooled and held between 80° C. to 90° C. as the mash liquefies. The mash is then cooled to 60° C. and additional enzymes may be added in a saccharification step. After a period of time at 60° C., the mash is cooled to ambient to ˜35° C., and the liquid is then sent to fermentors where yeast is added to convert sugars to ethanol. In a continuous process utilization of a number of serially linked fermentors is typical, as this is required for efficient conversion of the sugars and also because ethanol-production-favorable conditions (which depend on the amount of alcohol and other byproducts present in the mash) can be optimized.
The economic viability of the ethanol production process, and indeed whether the process results in positive production of fuel source, depends to a large extent on the recovery and re-use of the latent heat of the mash. This recovery and re-use of heat energy is performed by use of heat exchangers, which are typically installed in the process to maximize the recovery of high quality heat. A substantial problem faced by the industry is that in certain units, for example where mash is held at 90° C. to 150° C., most antimicrobial agents added to the mash would be inactivated and destroyed by the heat. In liquefaction and saccharification units operating at 90° C., this is only a minor problem as lactobacilli and other microorganisms cannot thrive at those temperatures. However, lactobacilli and other microorganisms can colonize and thrive in the heat exchangers held at lower temperatures at the exit of the higher temperature units. There is no practical method, using powdered antimicrobial agents, of providing an active concentration of the antimicrobial agent to mash passing through heat exchangers immediately following units operated at high temperatures. Antimicrobial agents added to units is largely inactivated by heat, and the kinetics of dissolution of these antimicrobial agents are too slow to provide an effective concentration of these antimicrobial agents in the time frame where the mash passes through the heat exchangers, so effective control of lactobacilli and other microorganisms in these first heat exchangers is not readily achieved using the most effective current antimicrobial products (pristinamycin-type antimicrobial agents, polyether ionophore-type antimicrobial agents, or both). In one aspect of this invention, use of presolubilized pristinamycin-type antimicrobial agents, polyether ionophore-type antimicrobial agents, or both, injected (by batch, pulse, or continuous means) immediately upstream of or even within the body of the heat exchangers, solves this problem.
Finally, the fermented mash (containing corn particles, alcohol, and water) is sent to a distilling column where alcohol is extracted, and the dried residual material find large markets in the animal feed business as DDG.
Large volumes are processed, and as one might imagine with all the temperature changes involved in the process that heat exchangers are critical to both net production of energy and to the economics of the process. Bacterial slime will impair heat exchanger efficiency. Similar use can be made in other areas of the plant, and in other unit operations in plants having different configurations, where there is a need to provide in a short period of time an effective concentration of pristinamycin-type antimicrobial agents, polyether ionophore-type antimicrobial agents, or both.
One particularly difficult problem is the control of competing microorganisms, in particular bacteria of the LAB (Lactic Acid Bacteria family) which encompasses many bacterial species of similar growth and physiological traits. One example is Lactobacillus paracasei which compete with the yeast for nutrients and produce lactic acid. Other microorganisms such as Acetobacter/Gluconobacter and wild yeasts must also be controlled. Since control of LAB is critical to the process viability and since control of one class of microorganisms by the methods described here results in control of at least some of the other microorganisms, this discussion will focus on LAB control. One of skill in the art will know that a number of other competing microorganisms will also be controlled by the treatment processes described here, depending on the antibiotics and antimicrobials used in the process. LAB bacterial contamination in the range of 106 to 107 per ml can reduce ethanol yield by 1-3%. LAB bacteria are present in all incoming carbohydrate sources, and are present in all areas of the ethanol production plant. In industrial processes such as the manufacture of ethanol for fuel, even with active control programs to control the proliferation of LAB bacterial, carbohydrate losses to LAB bacterial can range up to several % of the total carbohydrate input, which can make the difference between profitability and non-profitability. Further, if the lactic acid content of the mash approaches 0.8% and/or acetic acid concentration exceeds 0.05%, the ethanol producing yeast are stressed and yeast metabolism is reduced. In the manufacture of certain alcoholic beverages, the proliferation of lactobacilli and its byproducts can unfavorably alter the taste and value of the product.
One very effective control program involves the introduction of pristinamycin-type antimicrobial agents, and particularly virginiamycin, to the process. These pristinamycin-type antimicrobial agents, and particularly virginiamycin, are preferred because: 1) they are very effective against a number of microorganisms including LAB bacteriala at low concentrations, e.g., 0.3 to 5 ppm, 2) microorganisms do not tend to develop resistance to this type of antimicrobial agent, 3) the antimicrobial agent does not significantly hinder the yeast, and 4) the antimicrobial agent is effectively destroyed by the drying of the end “waste” product so that it is not introduced indiscriminately into the environment. Usually, the “waste” byproduct, known as “Dried Distillers Grains with Solubles” (DDGS), is sold as animal feed, going 45% to dairy, 35% to beef, 15% to swine, and 5% to poultry industries. This is an important factor in the profitability of an ethanol production process, and the total amount of this byproduct produced per year is on the order of 3.5 million metric tons per year. The presence of residual antimicrobial agents in this material can adversely affect the value of this byproduct, as small residual amounts of antimicrobial agents in feed will promote the development of agent-resistant microorganisms. We have tested DDGS samples from 8 major ethanol producers using virginiamycin to control microorganisms and found no detectable amount of virginiamycin in the DDGS (<1 ppm via the validated Eurofins analysis and <1 ppb via an unvalidated experimental analytical procedure).